1. Field of the Invention
The present invention relates to an optical encoder, and in particular to an optical encoder utilizing an optical grating as an optical scale.
2. Related Background Art
In information equipment such as an electronic typewriter, optical encoders are commonly employed for detecting the position and speed of moving parts such as carriages. Such an optical encoder is usually fixed on the moving part, and is so constructed as to project light onto an optical scale on which optical codes are recorded, and to photoelectrically convert thus modulated light with a photodetector through a fixed scale thereby obtaining the information on the position of said moving part as an encoded electrical signal. Said optical scale has been composed of:
(I) a metal plate in which slits are etched; or PA1 (II) a transparent substrate such as glass or plastic on which a metal layer such as of silver, copper, chromium or aluminum is deposited by evaporation and is etched to form slits.
Such conventional optical scales are however not suitable for recording very fine code patterns since the slit width obtainable with etching is limited to about twice the metal layer thickness. Also they are inevitably expensive as they require a complex manufacturing process and involve the use of expensive photosensitive resin for etching.
On the other hand, optical scales not associated with such drawbacks and optical encoders utilizing such optical scales have been proposed for example in the U.S. Pat. Nos. 3,598,493 or 4,536,650, as will be explained in the following.
FIGS. 1 and 2 illustrate a conventional optical encoder, wherein FIG. 1 is a plan view of an optical scale while FIG. 2 is a schematic cross-sectional view thereof. The optical scale 50 is composed of a transparent member having mutually parallel faces 51, 52, and, on the face 51 there are alternately formed flat light-transmitting portions 55 and non-transmitting portions 54 composed of V-sectioned recesses. Among the light coming from an unrepresented light source, the light L2 entering said non-transmitting portion 54 is totally reflected by the sloped surfaces and returns to the incoming direction. On the other hand, the light L1 entering the transmitting portion 55 passes said optical scale 50 and is photoelectrically converted by a photoreceptor 56. Consequently a relative displacement of said photoreceptor 56 and the optical scale 50 causes the photoreceptor 56 to generate pulse signals corresponding to the amount of said displacement.
However, in such a conventional optical encoder, it has been difficult to produce the optical scale with satisfactory precision. For example, the optical scale shown in FIG. 1 can be produced by first preparing a master of the same shape, then preparing an inverted mold from said master, and molding a transparent material with said inverted mold. Said master is usually prepared by forming recesses with a V-shaped punch on a metal plate. However the final form is often distorted from the design shown in FIG. 2 because of mechanical stress applied to the metal plate in the preparation of the master, and such distortion in the preparation gives rise to an aberration in the behavior of the incident light, thus leading to deterioration of the quality of the signal obtained from the photoreceptor. Also, even if the optical scale is prepared in the ideal form, the diffraction of light occuring at the edge of the recesses causes scattering in the light transmitted by the scale, thus lowering the S/N ratio of the detected signal. In FIG. 1, 53 indicates a mark for detecting a home position, formed in the same manner as the non-transmitting portions.